Cell: The Unit of Life

• Introduction & Cell Theory

  • Discovery
    • Anton von Leeuwenhoek: First observed live cells
    • Robert Brown: Discovered the Nucleus
  • Cell Theory
    • Schleiden & Schwann: Formulated initial Cell Theory
    • Rudolf Virchow: Added Omnis cellula-e-cellula (cells arise from pre-existing cells)
    • Core Postulates: All organisms consist of cells; Cell is basic structural & functional unit

• Prokaryotic Cells

  • General Features
    • Lack membrane-bound organelles & defined nucleus
    • Examples: Bacteria, Blue-green algae, Mycoplasma (smallest cell), PPLO
  • Cell Envelope
    • Glycocalyx: Outer layer (Capsule or Slime layer)
    • Cell Wall: Middle rigid layer; provides structural support
    • Plasma Membrane: Innermost semi-permeable layer
  • Internal Structures
    • Mesosomes: Membrane infoldings for respiration & cell wall formation
    • Chromatophores: Pigment-containing structures (in photosynthetic bacteria)
    • Ribosomes: 70S type (50S + 30S subunits)
    • Inclusion Bodies: Reserve material storage (e.g., Phosphate granules, Gas vacuoles)
    • Nucleoid: Genomic DNA (single circular chromosome, non-enveloped)
    • Plasmids: Small circular extrachromosomal DNA
  • Appendages
    • Flagella: Used for motility
    • Pili & Fimbriae: Used for attachment and conjugation

• Eukaryotic Cells

  • General Features
    • Presence of membrane-bound organelles & true nucleus
  • Plant vs Animal Cells
    • Plant Cells: Possess Cell Wall, Plastids, and large Central Vacuole
    • Animal Cells: Lack cell wall & plastids; possess Centrioles

• Cell Envelope & Transport (Eukaryotes)

  • Cell Membrane
    • Fluid Mosaic Model: Proposed by Singer and Nicolson (1972)
    • Composition: Phospholipid bilayer, proteins (Integral & Peripheral), carbohydrates
    • Transport Mechanisms:
      • Passive Transport: Diffusion & Osmosis (no energy required)
      • Active Transport: Against concentration gradient using ATP (e.g., $Na^+/K^+$ pump)
  • Cell Wall
    • Non-living, rigid outer layer in plants, fungi, and algae
    • Linked by Plasmodesmata (cytoplasmic connections between cells)
    • Composition: Cellulose, hemicellulose, pectins (Plants); Chitin (Fungi)

• Endomembrane System

  • Endoplasmic Reticulum (ER)
    • RER (Rough ER): Bears ribosomes; site of protein synthesis & secretion
    • SER (Smooth ER): Lacks ribosomes; site of lipid & steroid synthesis
  • Golgi Apparatus
    • Discovered by Camillo Golgi
    • Consists of flattened sacs called Cisternae
    • Has Cis face (forming/convex) and Trans face (maturing/concave)
    • Function: Packaging and modification of proteins and lipids
  • Lysosomes
    • Vesicles formed by Golgi apparatus
    • Rich in hydrolytic enzymes (hydrolases: lipases, proteases, carbohydrates)
    • Active at acidic pH ("Suicidal bags")
  • Vacuoles
    • Bound by a single membrane called Tonoplast
    • Stores water, sap, and excretory products
    • Contractile vacuole: Osmoregulation in Amoeba

• Energy Transducing Organelles

  • Mitochondria
    • Powerhouse of the cell: Site of aerobic respiration (ATP generation)
    • Double-membrane organelle; inner membrane forms infoldings called Cristae
    • Matrix: Contains circular DNA, 70S ribosomes, and metabolic enzymes
  • Plastids
    • Chloroplasts: Site of photosynthesis
      • Structure: Double membrane, Stroma, Thylakoids stacked into Grana
      • Contains Chlorophyll pigments
    • Chromoplasts: Store fat-soluble pigments (Carotenoids, Xanthophylls)
    • Leucoplasts: Colorless storage plastids
      • Amyloplasts: Store carbohydrates (starch)
      • Elaioplasts: Store oils and fats
      • Aleuroplasts: Store proteins

• Non-Membranous Organelles & Cytoskeleton

  • Ribosomes
    • Discovered by George Palade ("Palade particles")
    • Composed of rRNA and proteins
    • 80S type in Eukaryotes (60S + 40S subunits)
  • Cytoskeleton
    • Filamentous protein network (Microtubules, Microfilaments, Intermediate filaments)
    • Functions: Mechanical support, motility, maintenance of cell shape
  • Cilia and Flagella
    • Hair-like projections for cell movement
    • Axoneme: Core structure with a 9+2 array of microtubules
    • Emerge from centriole-like Basal Body
  • Centrosome and Centrioles
    • Contains two perpendicular Centrioles
    • Structure: 9+0 array of triplet microtubules ("Cartwheel" structure)
    • Function: Forms spindle fibers during cell division

• Nucleus & Microbodies

  • Nucleus
    • Nuclear Envelope: Double membrane with Nuclear Pores for transport
    • Nucleoplasm: Contains Nucleolus and Chromatin
    • Nucleolus: Site of active rRNA synthesis
    • Chromatin: Complex of DNA, basic proteins (Histones), and non-histones
  • Chromosome Classification (By Centromere position)
    • Metacentric: Middle centromere (equal arms)
    • Sub-metacentric: Centromere slightly off-center
    • Acrocentric: Centromere near the end (one extremely short arm)
    • Telocentric: Terminal centromere
  • Microbodies
    • Minute, membrane-bound vesicles containing enzymes (e.g., Peroxisomes)